1. Field of the Invention
The invention is related to microelectromechanical systems (MEMS).
2. Description of the Related Art
In general, microelectromechanical systems (MEMS) are very small mechanical devices. Typical MEMS devices include sensors and actuators, which may be used in various applications, e.g., resonators (e.g., oscillators), temperature sensors, pressure sensors, or inertial sensors (e.g., accelerometers or angular rate sensors). The mechanical device is typically capable of some form of mechanical motion and is formed at the micro-scale using fabrication techniques similar to those utilized in the microelectronic industry, such as using lithography, deposition, and etching processes.
In general, a MEMS transducer converts energy between different forms, e.g., electrostatic and mechanical forms. MEMS transducers may be used as both sensors that convert motion into electrical energy (accelerometers, pressure sensors, etc.) and actuators that convert electrical signals to motion (comb drive, micromirror devices, resonators). MEMS devices using capacitive transducers are easy to manufacture and result in low noise and low power consumption sensors and/or actuators.
Capacitive sensing is based on detecting a change in capacitance of a capacitor. If a known voltage is applied across the capacitor (e.g., fixed DC potential differences applied across the capacitors a the MEMS device), changes in current due to capacitive variations will appear in response to motion of one plate of the capacitor relative to another plate of the capacitor. Similarly, capacitive actuation is based on variation in electrostatic forces between the two plates of a MEMS capacitive transducer. For example, a DC operating point can be established by applying a DC bias voltage across the capacitor and an AC voltage causing changes in force on a plate of the capacitor. Transduction of a MEMS device is based on the voltage across the transduction gap generating an electrostatic force, or inversely, transduction based on the gap variation due to displacement generating a charge variation at the output of the transducer. The transduction gap may vary as a function of environmental factors (e.g., temperature, strain, and aging), thereby changing the capacitance with respect to time. These same environmental factors can also affect the spring constant (i.e., spring stiffness) associated with a MEMS device, which is typically modeled as a mass-spring-damper system. In general, a change in the electrode capacitance affects the equivalent spring stiffness through electrostatic pulling, which affects the resonant frequency of the MEMS device. MEMS devices targeting applications requiring high-precision (e.g., resonators having resonant frequency specifications required to be within +/−10 parts-per-million (ppm)) may not achieve the target specification due to effects of environmental factors on the resonant frequency.
A MEMS device may be configured as a resonator that is used in timing devices. The resonator may have a variety of physical shapes, e.g., beams and plates. The MEMS device may have a portion suspended from the substrate (e.g., a suspended mass, body, or resonator) attached to the substrate by an anchor. An exemplary suspended mass may be a feature such as, but not limited to, a beam, a plate, a cantilever arm, or a tuning fork. In a specific embodiment, a MEMS device includes a resonating feature (e.g., suspended mass) flanked by one or more drive electrodes and one or more sense electrodes.
Referring to FIG. 1, a conventional MEMS device (e.g., MEMS device 100) includes resonator 105 coupled to substrate 102 via anchor 104. During operation, electrode 110 electrostatically drives resonator 105 to dynamically deflect, which increases a capacitance between resonator 105 and electrode 110 when a voltage differential exists between resonator 105 and electrode 110 by decreasing the gap between resonator 105 and electrode 110. Since electrode 110 and resonator 105 are the same height and thickness and are in the same plane, resonator 105, when driven, deforms laterally, i.e., parallel to the plane of the substrate, across a distance between electrode 110 and a second electrode 111. Electrode 110 is substantially parallel to substrate 102. Electrode 111 detects the resonant frequency of resonator 105 as the capacitance varies between resonator 105 and electrode 111 in response to the deflection driven by electrode 110. MEMS device 100 is commonly referred to as an “in-plane” or “lateral” mode resonator because resonator 105 is driven to resonate in a mode where the resonator 105 moves laterally (in direction 109) and remains aligned vertically with electrode 110.
Referring to FIG. 2, in an exemplary MEMS application, MEMS device 100 is coupled to amplifier 210 in an oscillator configuration. Sense electrode 202 provides a signal based on energy transfer from a vibrating resonator of MEMS device 100, thereby converting mechanical energy into an electrical signal. In general, bias signals introduced at various points of the circuit determine an operating point of the circuit and may be predetermined, fixed DC voltages or currents added to AC signals. The resonator of MEMS device 100 receives a DC bias voltage, VMASS, which is generated by a precision voltage reference or voltage regulator of bias generator 206. However, in other embodiments, bias signals may be introduced at the electrodes and/or other nodes of the oscillator circuit. A large feedback resistor (RF) biases amplifier 210 in a linear region of operation, thereby causing amplifier 210 to operate as a high-gain inverting amplifier. The MEMS oscillator sustains vibrations of MEMS device 100 by feeding back the output of amplifier 210 to a drive electrode of MEMS device 100. Amplifier 210 receives a small-signal voltage from sense electrode 202 and generates a voltage on drive electrode 204 that causes the resonator of MEMS device 100 to continue to vibrate. MEMS device 100 in combination with capacitances C1 and C2 form a pi-network band-pass filter that provides 180 degrees of phase shift and a voltage gain from drive electrode 204 to sense electrode 202 at approximately the resonant frequency of MEMS device 100.
For some MEMS applications (e.g., a low-power clock source), a low-power, high-Q (i.e., quality factor), stable, and accurate oscillator may be required. However, the power, accuracy, and stability specifications may be difficult to achieve using the conventional MEMS device of FIG. 1. Accordingly, improved MEMS devices, e.g., MEMS devices that reduce or eliminate factors that affect accuracy and reliability of the output frequency of the MEMS device, are desired.